1. Field of the Invention
The present invention relates to a magnetic field detecting element that is used in a hard disk drive, and more particularly to a method for manufacturing a magnetic field detecting element.
2. Description of the Related Art
Spin-valve-type GMR (Giant Magneto-Resistive) heads are known in the art as magnetic heads that meet requirements for high sensitivity and high output. A magnetic field detecting element that is used in a spin-valve-type GMR head comprises a free layer and a pinned layer that are stacked one on the other with a nonmagnetic spacer layer sandwiched therebetween. The free layer is a ferromagnetic material layer whose magnetization direction is variable depending on an external magnetic field. The pinned layer is a ferromagnetic material layer whose magnetization direction is fixed with respect to the external magnetic field. The magnetization direction of the free layer forms a relative angle with respect to the magnetization direction of the pinned layer depending on the external magnetic field. Depending on the relative angle, a change in the spin-dependent scattering of conduction electrons is caused and, as a result, a change in magneto-resistance is caused. The magnetic head detects such a change in magneto-resistance so as to read magnetic information that is recorded in a recording medium.
The pinned layer may be constructed as a so-called synthetic pinned layer. The synthetic pinned layer comprises an outer pinned layer, whose magnetization direction is fixed with respect to an external magnetic field, an inner pinned layer, which is disposed closer to a spacer layer than the outer pinned layer, and a nonmagnetic intermediate layer that is sandwiched between the outer pinned layer and the inner pinned layer. The magnetization direction of the inner pinned layer is firmly fixed by antiferromagnetic coupling with the outer pinned layer. Further, since the magnetic moment of the outer pinned layer and that of the inner pinned layer cancel each other out, overall leakage of magnetic field is limited.
Many types of GMR heads are known. Of the many types of GMR heads, a CPP (Current-Perpendicular-to-the-Plane)-GMR head, in which sense current flows in a direction that is perpendicular to the film planes, has been studied because it is capable of obtaining stable output voltage even when it is used for a high-density recording medium having a narrow track width. The CPP-GMR head is also advantageous in that it has a high efficiency for heat radiation and accommodates a high operating current because of the structure in which the magnetic field detecting element and shield layers are connected to each other via metal layers. In the CPP-GMR head, the magnetic field detecting element exhibits a large resistance value and resultantly an increased output voltage in accordance with the decrease in the cross section thereof. Therefore, the CPP-GMR head is more suitable for a narrow track width.
The change in magneto-resistance becomes larger as the spin polarizability of the free layer and the pinned layer becomes larger. Therefore, if a material having large spin polarizability is used for the free layer and the pinned layer, then the magneto-resistance ratio (MR ratio), as well as output voltage, becomes larger. A magnetic material whose spin polarizability is 100% or nearly 100% is called “half metal”. Heusler alloy is known as one of the materials that realize the half-metal. In recent years, it has been proposed to use the Heusler alloy in the free layer and the pinned layer, rather than CoFe alloy and NiFe alloy that have heretofore been used. For example, JP-A 2003-218428 discloses a technology for using CO2MnZ (Z represents an element that is selected from the group consisting of Al, Si, Ga, Ge, and Sn) as a magnetic field detecting element in a CPP-GMR head.
If Heusler alloy is used in the free layer and the pinned layer, then heat treatment (annealing) at a relatively high temperature is required in order to provide regularization of crystal that is required to achieve half-metal characteristics. The temperature of the heat treatment is often no less than about 573K (300° C.). However, the magnetic field detecting element is covered with shield layers at its upper and lower surfaces in order to detect the magnetic field that is generated by a specific recording bit only of the recording medium. Because the temperature of the heat treatment is close to the upper limit temperature for ensuring that the nature of the shield layer is maintained, it is impossible to increase the annealing temperature far beyond the above-mentioned temperature if the Heusler alloy is used in the free layer and the pinned layer. Consequently, the Heusler alloy is not sufficiently regularized, and it is impossible to fully exploit the nature of the Heusler alloy.